Do increasingly depleted δ<Superscript>15</Superscript>N values of atmospheric N<Subscript>2</Subscript>O indicate a decline in soil N<Subscript>2</Subscript>O reduction?

Growing concentrations of N₂O within the atmosphere have been accompanied by decreasing δ¹⁵N values, provoking the hypothesis of a global decline in the rate of N₂O reduction relative to its production in soil. We estimate that the ratio of N₂O produced to N₂O reduced within the soil profile has declined by about 10–25% relative to its pre-industrial value. To a smaller extent, a reduction in the uptake of atmospheric N₂O at the soil surface relative to its emission could also have contributed to the reported isotopic signal. This calls for a greater consideration of the process of N₂O reduction in soil and its role in the global turnover of N₂O.     

Symbiotic relationships between soil fungi and plants reduce N2O emissions from soil

N₂O is a potent greenhouse gas involved in the destruction of the protective ozone layer in the stratosphere and contributing to global warming. The ecological processes regulating its emissions from soil are still poorly understood. Here, we show that the presence of arbuscular mycorrhizal fungi (AMF), a dominant group of soil fungi, which form symbiotic associations with the majority of land plants and which influence a range of important ecosystem functions, can induce a reduction in N₂O emissions from soil. To test for a functional relationship between AMF and N₂O emissions, we manipulated the abundance of AMF in two independent greenhouse experiments using two different approaches (sterilized and re-inoculated soil and non-mycorrhizal tomato mutants) and two different soils. N₂O emissions were increased by 42 and 33% in microcosms with reduced AMF abundance compared to microcosms with a well-established AMF community, suggesting that AMF regulate N₂O emissions. This could partly be explained by increased N immobilization into microbial or plant biomass, reduced concentrations of mineral soil N as a substrate for N₂O emission and altered water relations. Moreover, the abundance of key genes responsible for N₂O production (nirK) was negatively and for N₂O consumption (nosZ) positively correlated to AMF abundance, indicating that the regulation of N₂O emissions is transmitted by AMF-induced changes in the soil microbial community. Our results suggest that the disruption of the AMF symbiosis through intensification of agricultural practices may further contribute to increased N₂O emissions.     

Biosphere-atmosphere interactions of ammonia with grasslands: Experimental strategy and results from a new European initiative

A new study to address the biosphere-atmosphere exchange of ammonia (NH₃) with grasslands is applying a European transect to interpret NH₃ fluxes in relation to atmospheric conditions, grassland management and soil chemistry. Micrometeorological measurements using the aerodynamic gradient method (AGM) with continuous NH₃ detectors are supported by bioassays of the NH₃ `stomatal compensation point' (<sub>s</sub>). Relaxed eddy accumulation (REA) is also applied to enable flux measurements at one height; this is relevant to help address flux divergence due to gas-particle inter-conversion or the presence of local sources in a landscape.Continuous measurements that contrast intensively managed grasslands with semi-natural grasslands allow a scaling up from 15 min values to seasonal means. The measurements demonstrate the bi-directional nature of NH<sub>3</sub> fluxes, with typically daytime emission and small nocturnal deposition. They confirm the existence of enhanced NH<sub>3</sub> emissions (e.g. 30 g N ha<sup>–1</sup> d<sup>–1</sup>) following cutting of intensively managed swards. Further increased emissions follow fertilization with NH<sub>4</sub>NO<sub>3</sub> (typically 70 g N ha<sup>–1</sup> d<sup>–1</sup>). Measurements using REA support these patterns, but require a greater analytical precision than with the AGM.The results are being used to develop models of NH<sub>3</sub> exchange. `Canopy compensation point' resistance models reproduce bi-directional diurnal patterns, but currently lack a mechanistic basis to predict changes in relation to grassland phenology. An advance proposal here is the coupling of _s to dynamic models of grassland C–N cycling, and a relationship with modelled plant substrate-N is shown. Applications of the work include incorporation of the resistance models in NH₃ dispersion modelling and assessment of global change scenarios.     

Nitric oxide flux from soil during the growing season of wheat by continuous measurements of the NO soil–atmosphere concentration gradient: A process study

The surface flux of nitric oxide from a wheat field was investigated from 23 March to 29 May 1997 in the Kerzersmoos, Switzerland. A plot fertilised with 19 kg N ha^-1 in cattle slurry and 40 kg N ha^-1 in mineral NH₄NO₃ fertiliser and a plot receiving no nitrogen containing fertiliser were compared. The flux was calculated based on hourly measurements of the NO soil–atmosphere concentration gradient using the one-dimensional soil diffusion model of Galbally and Johansson (1989). The soil bulk diffusion coefficient was determined from measurements of the ²²²Rn surface flux and the activity gradient between 10 cm depth and the surface. It ranged between 79% and 0.3% of the NO diffusion coefficient in air and was parameterised by air filled soil pore space. The indirectly determined NO flux agreed well with standard flux measurements using dynamic chambers. The largest NO emission was found following fertiliser application and irrigation. The emission occurred in pulses, which lasted for 4 days up to 3 weeks coinciding with elevated soil ammonium concentrations. Nitric oxide emission in 5 days following application of cattle slurry were 31 g NO-N ha^-1 and 5 g NO-N ha^-1 from the non-fertilised plot, respectively. Nitric oxide emission in 15 days following application of NH₄NO₃ was 95 g NO-N ha^-1 and 10 g NO-N ha^-1 from the non-fertilised plot, respectively. NO emission in 4 days following irrigation on 21 April were 36 g N ha^-1 from the fertilised and 39 g N ha^-1 from the non-fertilised plot. The daily NO emission before and after fertiliser and irrigation pulses was between 0.3 and 0.7 g NO-N ha^-1 d^-1. NO production and NO uptake of the soil was measured regularly. No systematic influence of management or climate on NO uptake was found. NO production was strongly stimulated by fertiliser input and soil moisture content. The simulation of NO production could be reproduced using a nitrification algorithm (Riedo et al., 1998) driven by soil temperature, moisture and ammonium concentration. A NO production rate constant of 1.1ċ10^-3 h^-1 at 15 °C was derived from a linear regression between nitrification and NO production. Introducing the parameterisation of NO production into the model of Galbally and Johansson (1989) the duration and the strength of the NO emission pulses could be reproduced and the total NO emission during the experiment was approximated within a factor of two. This revised version was published online in June 2006 with corrections to the Cover Date.     

A comparison of repeated soil inventory and carbon flux budget to detect soil carbon stock changes after conversion from cropland to grasslands

Assessment of soil carbon (C) stock changes over time is typically based on the application of two methods, namely (i) repeated soil inventory and (ii) determination of the ecosystem C budget or net biome productivity (NBP) by continuous measurement of CO₂ exchange in combination with quantification of other C imports and exports. Here, we applied both methods in parallel to determine C stock changes of two temperate grassland fields previously converted from long‐term cropland. The grasslands differed in management intensity with either intensive management (high fertilization, frequent cutting) or extensive management (no fertilization, less frequent cutting). Soil organic C stocks (0–45 cm depth) were quantified at the beginning (2001) and the end (2006) of a 5 year observational period using the equivalent soil mass approach. For the same period and in both fields, NBP was quantified from net CO₂ fluxes monitored using eddy covariance systems, and measured C import by organic fertilizer and C export by harvest. Both NBP and repeated soil inventories revealed a consistent and significant difference between management systems of 170 ± 48 and 253 ± 182  g C m^?2 a^?1, respectively. For both fields, the inventory method showed a tendency towards higher C loss/smaller C gain than NBP. In the extensive field, a significant C loss was observed by the inventory but not by the NBP approach. Thus neither flux measurements nor repeated soil sampling may be suitable for tracking absolute changes in SOC, but both give similar answers with respect to relative changes.     

Betalactam antibiotics interfere with eukaryotic DNA-replication by inhibiting DNA polymerase alpha.

Betalactam antibiotics (BLA) are the most widely used antibacterial drugs in practical medicine. Recent experiments suggested that BLA, especially after "aging" in aqueous solutions, have an inhibitory effect on the growth of a variety of cultured human cells by interfering with DNA synthesis (Neftel et al. Cell Biol. Toxicol. 2, 513-521, 1986). Our initial observation that the replicative DNA polymerase alpha might be the target of the action of betalactam compounds (Hübscher et al. Cell Biol Toxicol. 2, 541-548, 1986) is now substantiated due to the following experimental data: (i) extractable DNA polymerase alpha is greatly reduced in cells that had been treated with BLA; (ii) the relative cellular distribution of thymidine and of its phosphorylated derivatives is not affected by BLA; (iii) BLA inhibit crude and highly purified mammalian DNA polymerase alpha; (iv) the inhibitory effect appears to be of the mixed type with a slight deviation from purely non-competitive behaviour towards the four deoxyribonucleoside triphosphates and; (v) the inhibition is evident in aphidicolin sensitive DNA polymerases from mammalian tissues and in DNA polymerases from DNA viruses such as Herpes simplex and Vaccinia. In sum, the results suggest that one of the most commonly used class of drugs has a target within eukaryotic cells being most likely the replicative DNA polymerase alpha.     

A new membrane tube technique (METT) for continuous gas measurements in soils

The metabolic activities of root nodules formed by Rhizobium tropici UM1899 were measured to test for the effects of geographical origin of the host bean (Phaseolus vulgaris L.) plant. Under increasing levels of N (0 to 24 mM of NH4NO3), the optimum condition for nitrogen fixation based on nitrogenase activity and allantoin concentration, was obtained between 2 and 4 mM N. Cultivars, including wild accessions from the two major domestication centers in America (Middle America and Andes), were then grown under aseptic conditions with 2 mM NH4NO3 and the rhizobial inoculant. Plant nodulins [leghaemoglobin (Lb), phosphoenolypyruvate carboxylase (PEPC) and glutamine synthetase (GS)], bacterial nitrogenase (NIF) activities as well as allantoin (ALA) concentration in the xylem sap, were assayed in flowering plants. Lb, PEPC, NIF activities and ALA concentrations were strongly affected by cultivar and by the center of origin. GS activity did not vary significantly with either cultivar or center of origin. LB, NIF and ALA were directly related to plant growth and offer opportunities to select for efficient N2-fixing symbioses. There were slight increases in nodulin activities of the domesticated cultivars, but the overall low variability within this material relative to landraces suggests that diversity for biological nitrogen fixation was reduced by domestication.